27. In the history of Europe, Ireland appears to have possessed – save for the interlude of the Ice Age – a far longer period of continuous vegetation than most parts of the Continent, stretching back in all probability (unless interrupted by the Chalk sea) to the uplift that took place in late Carboniferous times (11). In this place we are concerned only with the plants which occupy the country at present and their immediate predecessors; and the history of these began in the Pliocene period. But of the Pliocene flora in Ireland we know as yet nothing. The subsequent changes in the vegetation owing to the oncoming of the cold of the Pleistocene period have not so far been traced, nor have deposits yet been found which throw light upon the flora of the mild Interglacial period or periods which alternated with the Glacial phases. Recent research indicates that the Aurignacian mitigation of climate that preceded the final advance of the ice-sheet was of long duration, and that it was accompanied by a considerable uplift of the land. This would have offered opportunity for extensive re-migration into Ireland of plants driven out by the main advance of the ice, and probably many elements arrived at that time. In spite of geological evidence of the presence of ice in every part of Ireland, and the frequent assumption that at the period of maximum glaciation this ice was confluent over the whole country, leaving no portion unexposed, it does not seem by any means certain that the Interglacial immigrant plants did not find in the country survivors of the Glacial and Preglacial floras – rather the reverse. Nor is it certain how far the subsequent and final less severe glaciation may have reduced or exterminated the reconstructed Interglacial vegetation. The geological and the biological interpretations of the evidence relating to these questions are not as yet in accord on many points, and widely different views are still expressed regarding both the time and the mode of arrival of our present plants and our present animals*. Controversy on these points has raged particularly with regard to certain ingredients of the Irish flora which have their home in the Pyrenean and Mediterranean region on the one hand, and in North America on the other. These are specially referred to below (37), when the plants composing them are considered.

It is not disputed that a full reconstructed flora and fauna, little different from those which occupy the country at present, were domiciled in Ireland in early Postglacial times. Probably belonging to the beginning of this period, one may note the occurrence of Naias marina at L. Gur in Limerick in beds which yield remains of the extinct Great Irish Deer, which is assumed to have browsed a vegetation of tundra type. This plant is extinct in Ireland, and has now only one station in Britain (Norfolk). At Ballybetagh in Dublin Salix herbacea has been found in beds similarly associated with Cervus giganteus, at 750 ft. elevation, its nearest existing stations (in Wicklow) being at over 2000 ft*. A high land-level would seem to have persisted after the final passing away of the ice, allowing free re-colonization from the Continent. The break-down of the Irish-British land-connection prior to the disruption of a similar landbridge between England and the Continent accounts for the absence from Ireland of many common British plants (see 32) and animals – they were able to migrate overland into England, but found the way to Ireland blocked by the newly-formed Irish Sea.

* The following mostly recent papers on the Ice Age and its relations to the fauna and flora will illustrate some of the points of view: –

28. Since the separation of Ireland from England there have been minor fluctuations both of land-level and of temperature. The forests of Pinus and Betula which characterized early Postglacial times gave way to Quercus when a slightly higher temperature prevailed. A few records of plants belonging to these more recent times have been given by Erdtman*. The main growth of the peat bogs represents a somewhat colder and wetter phase than that which prevails at present. The white marl which so often underlies the bogs shows already a flora of present-day type*. An amelioration of climate in Neolithic times allowed the forests to extend up the flanks of the mountains several hundred feet above the present tree-limit, and permitted the growth of woods on western coasts and islands where now their descendants survive – if at all – as mere bushes. In quite recent times Quercus has supplanted Pinus completely; but previous to this Pinus would appear to have become rare owing to the encroachment of the all-conquering peat, the deterioration of the climate, and finally to grazing in pre-historic and historic times*. The peat bogs in their turn have now passed their zenith. On the mountains they are being denuded by wind and rain, and in the plain their increase has usually slackened or ceased, and the operations of man are steadily reducing their area, just as his influence has much lessened the area of lake and marsh in the country, owing to drainage.

From a European point of view, Ireland, on account of its western position and insularity, is the home of an intensely “Atlantic” flora.

THE ATLANTIC FLORA

29 “Atlantic Flora” is a relative term, dependent on the area which is being dealt with by the user of it, but signifying in all cases a flora developed under conditions of greater humidity and more uniform temperature throughout the year than those which prevail in adjoining areas. To the botanist of eastern Europe the flora of all the western European countries is essentially Atlantic. The central European worker might exclude E Spain, E France and much of Sweden from his Atlantic area, the indefiniteness of boundary being recognised by the addition of such terms as “Subatlantic” and “Pseudoatlantic.” For Britain, Watson's “Atlantic” type signifies “species chiefly seen in W England” ; half of the 62 species to which he awards full “Atlantic” status do not occur in Ireland, these being mostly of S English range, and the remainder have a quite irregular distribution in this country. The Irish botanist using the term “Atlantic” would in turn apply it to those 'plants which had in Ireland a range corresponding to the maximum climatic influence, within the island, of the neighbouring ocean – in other words, a W and S distribution. The focus of this flora is in Kerry, to which area many of the most interesting are confined (35); while some of its members of wider S and W range are:–

Thalictrum minus

Euphorbia hiberna

Arabis brownii

Juniperus communis

Draba incana

Juniperus sibirica

Saxifraga spathularis

Taxus baccata

Drosera longifolia

Neotinea intacta

Galium sylvestre

Sisyrinchium angustifolium

Asperula cynanchica

Naias flexilis

Hieracium iricum

Eriocaulon septangulare

Arctostaphylos uva-ursi

Sesleria caerulea

Gentiana verna

Adiantum capillus-veneris

Euphrasia salisburgensis

Asplenium viride

Bartsia viscosa

Polystichum lonchitis

This is by no means a group of “Atlantic” facies in a European sense. Half of these plants, while a few may be rather western inside Europe, extend beyond European confines into Asia and Africa; some are circumpolar. Three are members of the Lusitanian-Mediterranean migration along the W European coast (37); two belong to the N American immigration (37). On the British standard, the list contains only two “Atlantic” species, and while Watson's Atlantic plants are mostly southern in Britain, this list contains more of a northern than of a southern facies. From the point of view of distribution it is clear that the group in the list above is homogeneous only in one respect – its western range within Ireland. To many of the plants, the bare limestone “crags” of the Galway Bay area are the attraction; to others, the mountains; others again, truly Atlantic in a sense, are members of the Lusitanian and American immigrations.

30. The most genuine Atlantic group in the European (not American) sense consists of those whose world-range is confined to W Europe (a few extending to the Atlantic islands). Irish members of this group include :–

Ranunculus hederaceus

Ulex europaeus

Corydalis claviculata

Vicia orobus

Cerastium tetrandrum

Erica cinerea

Hypericum elodes

Wahlenbergia hederacea

Carum verticillatum

Digitalis purpurea

Conopodium majus

Scilla verna

Saxifraga hypnoides

This list, selected as representative of the plants exclusively W European in range, shows in Ireland, as might be expected, no general “Atlantic” tendency. They are equally spread on the E and on the W sides of this island. In Britain, 4 of the 13 belong to Watson's “Atlantic” type. This means that Atlantic conditions (in the Continental sense) being prevalent over the whole of Britain and Ireland, these plants can grow equally in E and W, so far as climate is concerned. But that even in Ireland some of them are near the limit of their temperature-range is shown by the behaviour of Ulex europaus arl Digitalis purpurea. The former extends N to Denmark, the latter to the Trondjem fiord in Norway. During hard winters in Ireland both suffer severely. In 1894-5 and again in 1916-7, in Wicklow and Down for instance, large quantities of both species were killed.

THE BRITISH AND IRISH FLORAS COMPARED

31. The present vegetation of Ireland, as has been seen, was derived largely by immigration from W Europe through Britain subsequent to the passing of the ice. We may now consider the results of this and any previous immigrations, as represented by the flora of to-day. The flora of. Britain is so well worked and so widely known that comparative notes as between the two islands are the simplest way of bringing out the salient features of the Irish flora.
The representation of total floras by numbers tends to be misleading on account of varying practice as to the validity of critical plants as species, subspecies, or varieties, and also as regards the inclusion or exclusion of alien plants of various degrees of permanency. On the conservative estimate used in “Cybele Hibernica” and “Irish Topographical Botany,” the flora of Ireland (species and subspecies of Phanerogams and Vascular Cryptogams, native or naturalized) may be taken as a little over 1000. The figure for Great Britain on the same basis is in the neighbourhood of 1500. On the basis of the “London Catalogue of British Plants,” 11th edition, the Irish total is slightly under 1300, the corresponding figure for Great Britain being 2300. The difference of proportion in these aggregate and segregate figures shows the greater extent to which critical plants have been worked out in the larger island.

British Plants Absent from Ireland

32. Great Britain extends through a range of latitude nearly three times greater than does Ireland (50° to 61° instead of 51i° to 554°) and N Scotland and especially the dry gravelly soils of S England support many plants unknown in Ireland, so it is not surprising to find in the latter area a smaller diversity of plants than in the larger island; but nevertheless the reduction in the Irish flora is due largely to its insularity: the Irish Sea has proved a serious barrier to immigration from the E, whence almost the whole of our flora is derived. This is clear from the fact that many common plants which extend from S England far up into Scotland have nevertheless failed to reach Ireland. Such for instance are :–

Genista anglica

89

(or 80 per cent.)

Ononis spinosa

71

63

Astragalus glycyphyllos

68

61

Lathyrus sylvestris

67

60

Chrysosplenium alterniflorum

79

71

Scabiosa Columbaria

72

64

Paris quadrifolia

77

69

Convallaria majalis ...

67

60

Avena pratensis

81

72

The numerals following the names show in how many of the 112 vice-counties into which H. C. Watson divided Great Britain each species is found. The majority of these plants have a rather wider range in England than in Scotland, thus approximating to the “British” range of the bulk of the Irish flora, from which they are nevertheless absent.

Common British Plants Rare in Ireland

33. A more subtle problem is presented by the case of a number of species which have succeeded in arriving in Ireland, but for some reason have not made good, being much rarer on the Irish than on the English side of the Channel. Some no doubt are recent arrivals (“recent” on a plant-migration time-scale), 'and will in due course extend their range. Others are more or less manifestly relict species, which for one reason or another are decreasing, and in the absence of reinforcements (one of the disadvantages of insularity) will die out. A few of the more striking cases are cited below. The numbers appended to the species express the number of Irish divisions (40 in all) and British vice-counties (112 in all) in which each occurs.

Plants rare in Ireland, widespread in BritainA. Plants extending from N to S in Britain.

No. of V.C.'s.

Same in % of V.C.'s.

Ireland

Britain

Irish

British

Trollius europaeus

3

53

7

47

Corydalis claviculate

6

96

15

88

Helianthemum chamaecistus

1

93

2

83

Geranium pratense

1

97

2

87

Hypericum hirsu turn

4

92

10

82

Astragalus danicus

1

47

2

42

Ornithopus perpusillus

5

86

12

77

Spiraea filipendula

2

65

5

58

Adoxa moschatellina

1

101

2

90

Galium cruciata

2

95

5

85

Calamagrostis epigejos

3

72

7

64

Cryptogramme crispa

7

55

17

50

Lycopodium inundatum

5

60

12

54

B. Plants of Northern Range in Britain

Geranium sylvaticum

1

54

2

48

Carex pauciflora

1

32

2

29

C. Plants of Southern Range in Britain.

Ranunculus fluitans

1

59

2

53

Teesdalia nudicaulis

4

82

10

73

Trifolium subterraneum

1

46

2

41

Trifolium glomeratum

2

21

5

19

Poterium officinale

4

69

10

62

Serratula tinctoria

1

68

2

61

Senecio erucifolius

5

69

12

62

Picris hieracioides

5

62

12

55

Hypochaeris glabra

1

55

2

50

Campanula trachelium

5

59

12

53

Limosella aquatica

2

53

5

47

Lamium galeobdolon

4

63

10

56

Colchicum autumnale

3

53

7

47

Phegopteris robertiana

1

29

2

26

The majority of the plants in these lists have in Ireland a continuous if limited range. In a few cases some reason edaphic, or climatic, or both – can be suggested for their restriction in this country, as in the case of three E coast species – Ornithopus, Trifolium subterraneum, and T. glomeratum; but most are puzzling. A few show in Ireland a widely discontinuous range – Paterium officinale (Mayo and the NE counties), Lycopodium inundatum (Kerry, Cork, Wicklow, Galway, Mayo). Such discontinuities of range are not uncommon in the flora in general, and are difficult to explain. In some cases they undoubtedly represent relict distribution.

Plants Commoner in Ireland than in Britain

34. Interesting also is the case of plants which, found both in Britain and in Ireland, have achieved greater success here than in the neighbouring island. The facts in regard to some of the more striking cases may be displayed as follows :–

Distribution

No. of V.C.'s.

Same in %

Irel.

Brit.

Irel.

Brit.

Irel.

Brit.

Spergularia rupicola

G

G

20

31

50

28

Lathyrus palustris

n

s

14

21

35

18

Rubia peregrina

S

SW

17

25

42

22

Andromeda polifolia

G

s

27

35

67

31

Orobanche hedera

s

S

27

23

67

21

Euphrasia salisburgensis

W

N

10

2

25

2

Pinguicula lusitanica

G

W

34

31

85

28

Utricularia intermedia

W

G

23

8

57

7

Euphorbia hiberna

S & W

SW

11

3

27

3

Scirpus filiformis

W & S

W & S

26

31

65

28

Rhynchospora fusca

SW & S

SW & S

20

12

50

11

Trichomanes radicans

W

W

15

5

38

4

Lastrea aemula

G

G

37

36

92

32

Equisetum trachyodon

N

N

16

1

40

1

Chara aculeolata

G

G

27

18

67

16

Chara desmacantha

G

G

24

13

60

12

G = General, N = northern, n = rather northern, and so on.

In the case of a few of these plants a reasonable suggestion can be advanced to explain their greater abundance in Ireland. Thus the higher humidity of air and soil may be held to account for the much wider range here of Trichomanes and Lastrea aemula, and the wide distribution of bog for Andromeda, Pinguicula lusitanica, Utricularia intermedia, Rhynchospora fusca. In other oases the advantages which Ireland offers as a habitat as compared with Britain are not obvious.

Irish Plants Absent from Britain.

35. One of the main interests that. Ireland offers to the botanical geographer lies in the presence of a number of plants, of diverse origin, which are absent from the neighbouring island. To put the matter in proper perspective we must add to these a few others, clearly of similar class, which are not wholly absent from Britain, though very rare therein – such as Euphorbia hiberna and Naias flexilis; and also one or two belonging to the same migrations, which have colonized Britain but failed to reach Ireland – notably Erica ciliaris and E. vagans. The Irish plants absent from Britain and their companions just referred to group themselves as follows :–

1. Plants of Pyrenean - Mediterranean Facies.

Ireland.

Britain.

Continent.

Saxifraga spathularis (umbrosa auct.)

S,E,W&NW

SW

Saxifraga geum

SW ct W

SW

Arbutus Unedo

SW & W

SW & S

Erica mediterranea

W

SW

Erica Mackaii

W

SW

Erica ciliaris

SW

SW

Erica vagans

SW

SW

Dabeocia polifolia

W

SW

Sibthorpia europtea

SW

SW & S

SW

Pinguicula grandifiora

SW

SW & Alps

Pinguicula lusitanica

G

SW & W.Scot.

SW

Euphorbia hiberna

S,W&NW

SW

SW

Euphorbia Peplis

S

SW&S

SW&S

Neotinea intacta...

W

S

Simethis planifolia

SW

SW

SW & S

Glyceria Foucaudii

SW &NE

S

S

The terms S, SW, etc., are here employed in a quite strict sense. SW Europe for instance means at most Spain, Portugal, and the W coast of France

2. Plants of N American Facies

Ireland

Britain

Continent

N.Amer

Spiranthes gemmipara

SW

-

-

NW

Spiranthes stricta

NE

W.Scot.

-

N

Sisyrinchium angustifolium

SW - NW

-

-

G

Juncus macer

S,W,& E

Intrd.

Intrd.

G

Juncus Dudleyii

-

Scot.

-

N

Naias flexilis

SW - NW

Scot.N.Eng.

N

G

Eriocaulon septangulare

SW - NW

W.Scot.

-

N

Fig. 7.
DISTRIBUTION OF THE PLANTS IN TABLES 1 AND 2.
Pinguicula lusitanica is omitted in the left-hand map, as it has a wide range in Ireland.

3. Other Species.

Ireland.

Britain.

Continent.

Arenaria ciliata

W

Arctic-alpine

Inula salicina

Shannon

Widespread, also Asia.

Chars tomentosa

Shannon and Westmeath

Widespread, also Asia, N.Africa

36. There are also a number of critical Irish plants which as at present known are absent from Britain, and some of them apparently endemic in Ireland; but the progress of field-work may yet reveal them in other places. Among these are :–

Arabis Brownii

Hieracium Scullyi

Alchemilla colorata

Hieracium Stewartii

Saxifraga Drucei

Hieracium subintegrum

Saxifraga Sternbergii

Orchis majalis

Saxifraga hirta

Equisetum Moorei

Saxifraga affinis

Equisetum Wilsoni

Saxifraga incurvifolia

Chara denudata

37. As regards the Pyrenean-Mediterranean group above, their curious range, mostly along the W coast, and their discontinuous distribution, have led many phytogeographers to assign a very early date for their arrival – either Preglacial, Interglacial, or early Postglacial. By others this is not admitted, and even quite recent arrival across existing seas has been advocated. It is impossible here to go into the pros and cons of a very debatable question; some of the leading contributions to the discussion of this subject have been given in a previous paragraph (27); but it should be borne in mind in all consideration of this matter that a fauna, small but varied, of similar distribution, is associated with the plants, and cannot be left out of account when the origin of the latter is being discussed. This fauna includes Mollusks (Helix pisana, Geomalacus maculosus), Beetles (Otiorrhynchus auropunctatus), False-Scorpions (Obisium carpenteri), Woodlice (Metoponorthus melanurus, M. cingendus, Philoscia couchii, Eluma purpurascens, Trichoniscus vividus), and Earthworms (Lumbricus friendi). The migration of these species across water-barriers offers much greater difficulties than in the case of the plants, and their Hiberno-Pyrenean or HibernoMediterranean range suggests early overland migration.
The American list can be materially reinforced in Scandinavia, where a number of additional northern North American plants, absent from W Siberia but present in Greenland, etc., are found; such are Ranunculus Cymbalaria, Rhododendron lapponicum, Campanula uniflora, Pedicularis fiammea, Carex scirpoidea, C. nadina. The occurrence of the American Dulichiunt spathaceum, now unknown in Europe, in deposits of the last Interglacial period in Denmark, and also in Poland, suggests that some at least of these plants were early arrivals. Naias flexilis also is a NW European Interglacial fossil.
The presence of the American group, which also includes at least one animal (the fresh-water sponge Heteronteyenia ryderi), offers great difficulties to any theory of recent or of trans-marine dispersal, and points to early migration, presumably via Iceland and Greenland, when the intervening seas were at least much narrower than they are at present. It should be remembered that in North America a corresponding group of European organisms is found along the NE coasts, pointing to reciprocal migration eastward and westward. Fur different views by geologists and biologists as to the time and mode of migration of these groups, the literature quoted in 27 should be studied.

38. Another element which makes its presence felt, especially on the W coast, which is the head-quarters of all the peculiar groups in the Irish flora, is the Arctic-alpine. The alpine flora of Ireland is in general poor, but in the W especially certain plants characteristically alpine on the Continent become abundant down to sea-level (66-69). Dryas octopetala (Plate 20), Arctostaphylos Uvaursi, Gentiana verna (Plate 23), Euphrasia salisburgensis (Plate 27) may be mentioned as examples.
And so, when studying the flora of W Ireland, we are dealing with a startling mixture of types, not to be found elsewhere in the British Islands or in Europe. The pool from which we gather the American Eriocaulon is fringed with Pyrenean Erica. The cracks which are ,filled with the delicate green foam of Adiantum (Plate 24) are set in Arctostaphylos and Gentiana, verna; Neotinea intacta, far from its Mediterranean home, sends up its flower-spikes through carpets of Dryas (Plate 2); and Dabeocia and Juniperus sibirica straggle together over the rocky knolls.

Relict and Incipient Species.

39. Assuming, as seems justified, that very limited range mostly signifies either incipient or relict distribution, we may analyse the single-station plants of Ireland, and by taking cognisance of their general range in Europe, attempt to determine where are the head-quarters from which they are advancing or the last refuges to which they are retreating; the single-station plants we may believe to be either the latest corners, or else the last remaining indication of former wider range. For this purpose we leave out of account plants in any way critical, of which the full range may not yet be determined: also any which are under suspicion of owing their presence to man. We find that of 21 plants so selected . . .

twelve are northern in their general range, namely:–

Arenaria ciliata

Saxifraga nivalis

Geranium pratense

Epilobium alsinefolium

Geranium sylvaticum

Pyrola rotundifolia

Astragalus danicus

Carex pauciflora

Lathyrus maritimus

Carex fusca

Rubus chamaemorus

Carex magellanica

Only four are southern:-

Trifolium subterraneum

Euphorbia Peplis

Erica Mackaii

Simethis planifolia

And five have a general range:-

Ranunculus fluitans

Scirpus triqueter

Helianthemum chamaecistus

Phegopteris robertiana

Adoxa. Moschatellina

If we take plants of somewhat wider Irish range, which either occupy a rather limited continuous area or have not more than two or three separated stations, we add 35 more. Of these nine have a distinctly northern range outside Ireland:–

Arabis petraea

Carex paradoxa

Potentilla fruticosa

Carex elongata

Alchermilla alpina

Calamagrostis epigejos

Pyrola secunda

Poa alpina

Melampyrum sylvaticum

while no less than 16 are of southern type:–

Matthiola sinuata

Erica mediterranea

Helianthemum guttatum

Dabeocia polifolia

Helianthemum canum

Microcala filiformis

Arenaria verna

Sibthorpia europma

Trifolium glomeratum

Pinguicula grandiflora

Saxifraga geum

Colchicum autumnale

Diotis maritima

Carex divisa

Arbutus unedo

Glyceria foucaudii

leaving six which are neutral in this respect:–

Elatine hydropiper

Ajuga pyramidalis

Spiraea filipendula

Asparagus maritimus

Inula salicina

Glyceria borreri

The first northern list, and to a less extent the second, is rich in alpine plants which are clearly relict from Glacial or Subglacial times. “Disintegration of area” is in some cases – e.g., Potentilla fruticosa – evident far beyond their Irish territory*. Presumably the Neolithic climatic optimum seriously affected them, and many are now in Ireland on the verge of extinction – for instance, Lathyrus maritimus, Rubus chamaemorus, Saxifraga nivalis, Carex fusca (? already gone) – whose solitary Irish station can in each case now be reckoned in square yards. Other plants whose Irish station or stations are far removed from the nearest adjoining one, whether in Ireland or in Britain – e.g., Astragalus danicus, Epilobium alsinefolium, Calamagrostis epigejos – are no doubt also relict. But some others, whose Irish stations adjoin those in Britain, are probably incipient natives of recent arrival. The best examples are a NE group of Scottish immigrants – Geranium pratense, G. sylvaticum, Melampyrum sylvaticum, etc. (See also 43.) The Dublin area supplies good similar examples in Senecio erucifolius and Lamium galeobdolon, and some of the south-eastern plants, such as Juncus acutus, also suggest incipient colonization.

* See for instance G. DRUCE in New Phytologist, 1913, 245.

The same explanations suggest themselves as regards the southern groups. Those members of the SW or Lusitanian group which are included in the table in 35 are to my mind clearly relict. Vanishing species also are Euphorbia peplis (apparently gone), Matthiola sinuata, Diotis maritima. Probably relict, in view of the wide gap between their Irish and next stations, are also Helianthemum chamaecistus, H. guttatum, H. canum, Glyceria foucaudii, etc. Beyond these few suggestions it does not seem very safe to go.

In viewing these facts, it should be remembered that a number of other species have a wider range in Ireland than in Britain, whence they must have come to us. A list of some of these is given elsewhere (34). Whether the present disproportion is due to decrease in England or to more suitable conditions prevailing in Ireland is difficult to guess. Possibly they should rank in Ireland as incipient species in an advanced stage of success, possibly as decaying species in Britain, possibly as both.

40. SUMMARY. – We may summarize the foregoing paragraphs as follows:– the leading feature of the Irish flora as compared with that of Britain is its great reduction, about one-third of the flora of Britain failing to re-appear in Ireland. Among those plants which have colonized Ireland, there are notable examples of species which have not succeeded, and appear near extinction, or which, arriving recently (in a comparative sense), have not yet had time to extend their range. There are others which have achieved conspicuous success in Ireland, so that they are here actually or relatively more abundant than in Britain. Finally, there is a number of species in Ireland not found in Britain. These belong especially to southern (mostly Pyrenean) and to north-western (N. American) immigrations, both of which have affected also to a slight extent the flora of Britain and the adjoining parts of Europe.

DISTRIBUTION OF PLANTS WITHIN IRELAND

41. In Ireland two general types of distribution govern the flora*. The first is primarily edaphic in character, caused by the general geological structure of the country – a central plain of limestone surrounded by groups of hills formed of non-calcareous rocks. This produces a Central and a Marginal type. The Central plants are largely calcicole species, inhabiting dry ground, marshes, and lakes; a few belong to the great peat bogs. Characteristic members of this group are:–

An interesting feature in connection with this group is the tendency of a number of its members to extend their boundaries in a NE direction, forming an arm as far as Lough Neagh. This appears to be due to an absence of hill-barriers in that direction, combined with NE extension of the Carboniferous limestone almost to the shores of that lake. Similarly a few penetrate SW into Kerry.

The plants of Marginal distribution (other than halophytes, which have no choice), while of homogeneous range, are of heterogeneous habitat; they comprise many alpine and montane species, and many of calcifuge tendency; also some xerophytes for which the light or sandy soils found near the sea are an attraction. Representative species are :–

Cerastium tetrandrum

Filago minima

Hypericum elodes

Hieracia

Radiola linoides

Jasione montana

Erodium moschatum

Scleranthus annuus

Trifolium striatum

Salix herbacea

Trifolium arvense

Carex dioica

Vicia sylvatica

Milium effusum

Sedum roseum

Lycopodium alpinum

Callitriche intermedia

Nitella translucens

42. The .other general type of distribution, which is partly due to topography, partly in all probability to the general direction of Postglacial immigration, and largely climatic, tends to produce isophytic boundaries running NE and SW, as suggested by fig. 8. In this connection fig. 4 should also be borne in mind, where both summer and winter isotherms are seen to follow similar directions, at least on the W coast. Precipitation also, in a broad sense, follows corresponding lines (fig. 6). See 47.

North – western Plants.

The plants of general northern and alpine range (Watson's “Scottish” and “Highland” types, the former especially), while concentrated in northern Ulster, run down the W coast in much larger numbers than down the E, though the E possesses plenty of high ground and a lower winter temperature. Examples of such range are:–

Watsonian type.

Irish range.

Silene acaulis

H

Derry, Don., Leitr., Sligo, Mayo.

Saxifraga oppositifolia

H

Derry, Don., Leitr., Sligo, Mayo, Galway.

Saxifraga aizoides

H

Antrim, Don., Ferm., Leitr., Sligo.

Callitriche autumnalis

S

Mainly in the N. and NW.

Circa alpina

S - H

Ulster (exc. Cavan and Mon.), Louth, Leitr., Sligo.

Hieracium lasiophyllum

Down, Antrim, Derry, Don., Ferm., Galway.

Pyrola media

S

Down, Antr., Derry, Tyr., Ferm., Don., Sligo, Leitr., Mayo. Clare.

Lamium molucellifolium

S

Mainly in the N. and NW.

Salix phylicifolia

S - H

Antr., Derry, Don., Leitr., Sligo, Mayo.

Potamogeton filiformis

S

N. half of Ireland, mainly western.

Very few “Scottish” or “Highland” plants can be found which prefer the E coast.

43. The presence in Ulster only (with head-quarters in Antrim) of a number of species mostly “Scottish” in their distribution in Britain, strongly suggests direct and probably recent colonization from Scotland. Examples are :–

Watsonian type.

Irish range.

Ranunculus fluitans

English

Antrim.

Trollius europmus

Scottish

Donegal, Fermanagh.

Geranium sylvaticum

Scottish

Antrim.

Geranium pratense

British – Eng.

Antrim.

Rubus chamaemorus

Highland

Tyrone.

Ligusticum scoticum

Scottish

Don., Derry, Antrim, Down.

Pyrola secunda

Scottish

Ferm., Antrim, Derry.

Melampyrum sylvaticum

Scottish

Antrim, Derry.

Equisetum pretense

Scottish

Ferm., Don., Antrim.

On the E side of Ireland we find no corresponding group of “English” type plants of limited range save species confined to the coast. The bulk of the “English” and “British” plants have swept right across Ireland. The “British” plants are everywhere, each of the forty Irish Divisions possessing 90 to 100 per cent. of the species present in this country. The “English” plants, also very widespread, show some tendency to decrease towards the NW, especially in inland areas.

44. As regards the maritime flora, it is worthy of note that a number of plants which are by no means confined to the coast in Britain are essentially or exclusively maritime in Ireland. These are largely but not entirely “English” plants, widespread especially on the light soils which prevail in S England. In Ireland they are found mainly on the E side (“E” in the last column of the succeeding table), but some range round the whole coast (“EW”). Those marked * have also one inland station, and those marked ** two, mostly on lake shores. The second column indicates the Watsonian type of distribution in Britain.

Cerastium semidecandrum

BE

EW

*Sagina subulata

SB

W

Trigonella ornithopodioides

E

E

Trifolium subterraneum

E

E

Trifolium arvense

BE

EW

Trifolium striatum

E

E

Trifolium scabrum

E

E

Trifolium glomeratum

E

E

**Trifolium fragiferum

E

EW

Ornithopus perpusillus

BE

E

Vicia lathyroides

BE

E

Oenanthe lachenalii

EB

EW

Plantago coronopus

B

EW

**Carex distans

B

EW

45. There is a group of East Coast species, largely maritime, which tends to range S rather than N. Exclusive of these, there is a fairly well-marked group with headquarters in the S, which tends to range up the E coast more than the W. Examples of the latter group are :–

Watsonian type

Irish type

Linum bienne

A - E

Kerry to Boyne, Limk., Clare.

Wahlenbergia hederacea

A

Kerry, Cork, Wexf., Wickl., Dubl., Mayo.

Erythraeea pulchella

E

Cork, Waterf., Wexf., Dubl., Leix.

Orobanche major

S - A

Cork, Wexford, Wicklow.

Salvia horminoides

G - L

Coast Cork to Louth, Kilk., Clare.

Juncus acutus

E - A

Cork, Waterf., Kilk., Wexf., Wickl.

It might be thought, in view of what one might call super-Atlantic conditions which find their maximum in Kerry and W Cork, whence they continue with slowly diminishing intensity up the W coast, that a marked group of plants of corresponding range would be found. This is true only in a general sense. The Lusitanian and American groups as a whole show this range, but individually they display considerable diversity of distribution. Conditions all along the W coast are sufficiently similar to allow a wide dispersal of the W coast species, and as a whole the plants southern in Ireland tend to range up the E rather than the W coast.

46. If we analyze the Irish flora according to. Watson's well-known Types of Distribution we do not get far. His classification, based on the range of plants in Great Britain, fails in many respects if applied to Ireland. His “English” group is irregularly spread in this country, showing a general SE trend; “Germanic” plants are almost non-existent here, as might be expected from the definition; the “Scottish” plants are in Ireland northern and western, the “Highland” species the same, but more western and less northern, while “Atlantic” species range all round the coast of Ireland, being rather more plentiful in the S half. But if we take Watson's “English” type as representing broadly the southern element in the Britannic flora, and his “Scottish” plants as representing the northern, we find that the limits of these groups in Britain and Ireland may be represented by rather suggestive curves as shown in the accompanying diagram (fig. 8).

47. This general NE-SW trend represents a feature which recurs in many connections. It is the direction of the ancient “Caledonian” folding which produced the leading topographical features of Scotland and of much of Ireland; and a glance at a geological map shows that it suggests also the main boundaries of the rocks of the southern half of England. These physical features are reflected in the distribution of the vegetation. Meteorological maps will show that it has besides a considerable climatic significance (see figs. 4, 7). It represents also the main front along which invasion advanced from the SE after the passing away of the ice, spreading W and N from a land-connection in the region of the Straits of Dover, and another across the Irish Sea. Its prevalence in the distribution of the flora is well shown for Britain in the maps illustrating J. R. Matthews' papers, and for Ireland in a paper of my own.